In the last decades, the interest toward wireless communications has greatly increased. Such an interest has pushed the development and refinement of wireless protocols and technologies. All types of wireless communications have one thing in common: they allow data transmission over the air. However, transmitting data over the air introduces issues such as interference, distortion and multipath. To overcome such issues, multiple techniques for treating received data signals have been developed in combination with more robust modulation techniques. Some of the mostly used modulation techniques include Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiplexing Access (OFDMA).
More particularly, OFDM and OFDMA modulation techniques include embedded mechanisms to reduce the effects of multipath fading and Intersymbol interference. For example, adjustable guard intervals and introduction of cyclic prefixes in the guard intervals are used to alleviate respectively the effects of multipath fading and Intersymbol interference. Although these mechanisms prove helpful, there still remains numerous challenges with reception and handling of an OFDM signal such as linear phase error and sampling error.
Many prior art documents propose methods and apparatuses for improving the quality of data received over the air using modulation techniques such as OFDM and OFDMA. Such documents generally propose techniques for profiling the incoming signal so as to perform synchronization, adjustment, interpolation and correction. Typically, such profiling is performed while the incoming signal is being decoded.
The article titled “Timing Recovery for OFDM transmission” published in the IEEE Journal on selected areas in communications, volume 18, number 11, in November 2000 by Baoguo Yang et al. describes a two-step method profiling. The first step of this method relies on autocorrelation of the received data, and more particularly the cyclic prefixes. Once the cyclic prefixes have been located, a second step is performed in the frequency domain. Performing profiling in the frequency domain is quite complex, adds latency to the decoding process, and typically requires more power, which is not interesting for wireless applications, such as, for example, Wireless Broadband (WiBro). Furthermore, this article does not address the issues related to correction of linear phase error and sampling error.
U.S. Pat. No. 6,959,050, granted on Oct. 25, 2005 to Motorola, Inc describes a method and an apparatus for profiling while synchronizing an OFDM signal in time, frequency and per-subcarrier rotation. More particularly, that patent also describes a two-step process, in which the first step consists of performing symbol timing synchronization and fractional frequency synchronization in the time domain. Afterwards, the second step of the process proceeds with performing per-subcarrier rotation synchronization in the frequency domain. However, this patent does not address or resolve issues related to correction of linear phase error and sampling error.
There is therefore a need for a method and an apparatus for correcting linear phase error of an OFDM signal.